Rock drill bits of the rolling cutter cone type are widely employed in such earth boring and drilling operations as the exploration for oil. Many of the earth formations encountered during these operations are quite hard and subject the drill bit to severe stress. Moreover, the drill hole is usually quite narrow in diameter at the bottom in comparison to the diameter of the top, and this configuration subjects the bit to forces from several directions during the course of drilling operations.
The drill bits most suitable for this kind of drilling usually include three cutter cones rotatably mounted on journals attached to a bit body so that the axis of each cone is oriented near the center of the bit, and the teeth located in concentric rings on the face of each cone intermesh with the teeth on adjacent cones to provide the chipping and crushing action required to cut through the earth formation being drilled and create the drill hole. The typical bit body employed to support the cutter cones is rotatably attached to one end of a drill pipe and includes a body portion with three depending leg sections, each of which has bearing and support structure for rotatably mounting a cutter cone which is secured to a journal.
While rolling cone bits have many advantages, a significant disadvantage of this type of drill bit is the inability of the cutter bearing and support structures to withstand the adverse directional forces encountered during drilling operations. In addition, the mounting and support structures themselves must be wear resistant and must hold the cones in the correct orientation. Rock bits are typically subjected to severe impacting and vibration as well as other wear inducing factors that are highly detrimental to the service life of the rotary cutter cone bearings and other bit components. At times, much of the weight of the drill pipe to which the bit body is connected may be caused to act upon the cutter cones, subjecting the cutter cones and their support structure to tremendous mechanical loads.
For example, during reaming operations a cutter cone will be thrust inward toward the center of the drilled hole. At other times, however, the cutter cone will be subjected to stresses that will thrust it away from the center of the hole. If the cutter cone is not held securely in place on its support structure on the bit body it will be thrust off the bit and lost in the drill hole. The loss of only one cutter cone usually is sufficient to render the drill bit essentially useless, and drilling operations must be halted while the bit is brought to the surface, and, if the cutter/cone assembly cannot be repaired or replaced, a new bit is installed. The loss of a cutter cone may not be detected for some time by the drill operator, and the continuation of drilling operations without a full complement of cutter cones could irreparably damage the remaining drill bit components. Moreover, the loss of a cutter cone is potentially extremely costly, since not only must drilling be stopped for an indeterminate time to bring the bit to the surface, but the lost cone, one or more other cones and possibly the entire bit must also be replaced, usually at considerable cost. Consequently, it is of the utmost importance that a rock drill bit be provided with cutter cone bearing and support structure capable of locking the cone in place in the presence of the high forces and other stresses to which rock drill bits are constantly subjected.
Prior art structures designed to support and retain the cutter cones on the bodies of rock drill bits have typically employed a journal bearing pin with bearing elements for rotatably mounting each cutter cone and means for retaining the cutter cone on the journal bearing pin. One commercially acceptable method of cutter cone retention requires the use of balls disposed in axially aligned circumferential grooves formed within the bearing surface of the pin and a corresponding cavity in the cone. This arrangement requires a ball passage extending from exteriorly of the pin to a circumferential groove to permit the loading of the balls after the cone is positioned on the pin. The balls occupy substantial axial space that might otherwise be used for greater journal bearing surface and capacity. Moreover, during normal drilling operations the cutter is forced inward toward the center of the hole so that the balls are directly subjected to loading which may result in the formation of metal debris within the bearing cavity from spalling or partial failure of the ball grooves. Ultimately, the debris may cause failure of the journal bearing and the retention mechanism so that the cutter cone is lost.
Other arrangements for supporting cutter cones which do not employ ball bearings have also been suggested. For example, U.S. Pat. Nos. 4,236,764 to Galle; 4,344,658 to Ledgerwood, III; and 4,491,428 to Burr et al all disclose a frictional cutter cone bearing and retention structure which includes a single substantially circular cross-section snap ring which is received in corresponding retainer grooves having a specifically defined configuration in the bearing pin and the cone to hold the cone on the pin. When the cutter cone is thrust inward, the ring is forced into the retainer groove in the cutter cone to secure it in place on the bearing pin. In arrangements like this which employ a single cutter cone retention snap ring, when the cone is thrust inward, as during reaming, the loading of the snap ring causes stresses that tend to urge the ring deep into the groove, with the possibility of cone loss. Moreover, the bearing capabilities of this kind of arrangement are very limited, and the cone is not securely retained when the cutter cone is subjected to outward thrust.
Retention structure for permanently retaining a cutter cone on a journal pin of a rock drill bit is disclosed in U.S. Pat. No. 4,511,108 to Brunson, wherein a single snap ring having a rectangular cross-section is located in corresponding grooves in the cone and journal pin. The size of the pin groove permits the ends of the ring to overlap and is also larger than the cone groove to prevent out-thrust loading on the ring so that ring is only intermittently subjected to thrust loading. However, despite its apparent advantages, the cone retention arrangement shown in this patent reduces the axial length of the journal bearing surface, thereby minimizing the journal bearing capability. It also requires precise machining of the pin to form a double width portion of the groove precisely where required to allow correct overlap of the snap ring when the cone is installed. Additionally, the pin must be correctly positioned on the bit body or proper insertion of the ring will not be possible, and the cutter cone is likely to respond adversely to thrust loading. Consequently, unless the journal is correctly attached to the bit body with the double width groove in the proper location, the ring may not retain the cone under the types of loads to which the cone is subjected during drilling so that cone loss and its attendant costs are likely.
U.S. Pat. No. 4,157,122 to Morris discloses a friction bearing assembly for a rotary earth boring drill bit wherein the cone cutter is mounted on a separate radial bearing element on the journal spindle. A single, optional retainer element shown to have a rectangular cross-sectional configuration, is also disclosed for use in enhancing cone retention on the bearing in drilling operations where the drill bit will be subjected to heavy loads and excessive vibration not likely to be withstood by the frictional engagement which holds the cone on the bearing. This arrangment, however, relegates to the bearing the dual function of securing the cone and bearing the weight of the cone rotation, which will accelerate bearing wear and, thus cone loss. Moreover, this assembly is not designed to respond to thrust loads on the cutter cone in opposite axial directions. Consequently, failure of the bearing assembly is likely to occur with the resultant loss of a cutter cone during heavy drilling operations or during the drilling of hard rock earth formations.
U.S. Pat. No. 4,444,518 to Schramm et al teaches retention means for a rock bit cone. This arrangement, while effective, employs a complex configuration of segmented arcuate pieces and separate spring elements to overcome the disadvantages of a single snap ring. These elements must be positioned around a circumferential groove in the journal to engage a corresponding groove in the cone and, therefore, are extremely time-consuming to assemble properly and, consequently, costly. Moreover, because significant axial bearing length is consumed by this arrangement, and it is not capable of absorbing axial thrust in both directions, the cone retention structure shown in this patent is not likely to withstand the repeated stresses resulting from most types of drilling operations to avoid cone loss.
The maintenance of the bit bearing structures in a properly lubricated condition is essential to the continued operation of the bit during drilling. Unless adequate lubrication is provided to the journal bearing surface, particularly when frictional bearings are employed, a cone will not be able to rotate freely. This may prevent the rotation of adjacent cones since the teeth will not intermesh properly and is likely to result in the premature termination of the drilling operation. However, none of the aforementioned patents discloses structure which ensures that adequate lubricant will be constantly supplied directly to the cone bearing structures to avoid these adverse consequences.
Therefore, the prior art fails to disclose cutter cone retention and bearing structure for rotary cone-type rock drill bits including separate bearing and retaining elements which provides maximum axial bearing surface and is capable of absorbing axial thrust in both directions and to retain the cutter cone securely in place during repeated, high stress drilling operations, or a cone lubrication supply system which directs lubricant where it is needed most to assure proper lubrication of the cone bearing structure during drilling.